Dual redox-active porous polyimides as high performance and versatile electrode material for next-generation batteries

Energy storage will be a primordial actor of the ecological transition initiated in the energy and transport sectors. As such, innovative approaches to design high-performance electrode materials are crucial for the development of the next generation of batteries. Herein, a novel dual redox-active and porous polyimide network (MTA-MPT), based on mellitic trianhydride (MTA) and 3,7-diamino-N-methylphenothiazine (MPT) monomers, is proposed for applications in both high energy density lithium batteries and symmetric all-organic batteries. The MTA-MPT porous polyimide was synthesized using a novel environmentally-friendly hydrothermal polymerization method. Rooted in its dual redox proprieties, the MTA-MPT porous polyimide exhibits a high theoretical capacity making it a very attractive cathode material for high energy density battery applications. The cycling performance of this novel electrode material was assessed in both high energy density lithium batteries and light-weight symmetric all-organic batteries, displaying excellent rate capability and long-term cycling stability.

Limited. All chemicals and solvents were used as received, without further purification.

IR spectroscopy measurements were recorded with a Bruker Tensor 27working in ATR
MicroFocusing MVP-QL with a diamond crystal. Resolution was set to 4 cm -1 and spectra were recorded from 4000 cm -1 to 600 cm -1 . The data was processed with Spectragryph optical spectroscopy software.
Thermogravimetric analysis was carried out using a Netzsch TG 209 analyzer at a heating rate of 10 K min -1 under nitrogen atmosphere, equipped with NETZSCH Proteus (Version 4.3) software.
Powder X-Ray diffraction was performed using a PANalyticalX'Pert Pro multi-purpose diffractometer (MPD) in Bragg Brentano geometry operating with a Cu anode at 45 kV, 40 mA and an X-Celerator multichannel detector. Samples were ground and mounted as loose powder on silicon single crystal sample holders. The diffraction patterns were recorded between 1 and 30° (2θ) with 74.970 s/step and a step size of 0.0201°. The sample holders were rotated during the measurement with 4 s/turn. Powder X-ray diffraction (pXRD) measurements of the powders were also carried out in a Bruker D8 Discover, with a IµS microfocus X-ray source (Cu-Kα radiation) at 50 kV and 100 mA, equipped with energy dispersive LynxEye detector. The samples were measured with 2s per steps, and step size 0,02°, and the samples oscillated in a range of 0,15mm in x and y (not rotated).

Supporting information
Scanning electron microscopy images were acquired in a field emission scanning electron microscope (SEM) Analytic SEM Zeiss Gemini 500 microscope, using 5kV acceleration. Prior to the analysis, the powders were sprinkled over a carbon tape, and the samples were coated with a 17 nm layer of Au/Pd 60/40 alloy in a Safematic CCU-010 LV Low Vacuum sputtering device.
Gas sorption Low pressure CO 2 physisorption isotherms were measured volumetrically at 195 K and 273 K up to 1 bar using an Autosorb-IQ-MP from Quantachrome equipped with a Quantachrome CryoCooler for temperature regulation. Isotherm points chosen to calculate the BET surface area were subject to the consistency criteria detailed by Rouquerol. 1 The pore size distribution was derived from the adsorption isotherms at 273 K using the Monte Carlo model for carbon adsorbents. Electrochemical characterization: Lithium metal batteries, based on MTA-MPT cathode, were assembled inside an argon glovebox using a lithium electrode (Gelon Supporting information energy Co. Limited, 11 µm thick) and a cathode electrode of a diameter of 11 mm. Glass microfiber filter (Whatman TM , GF/B, 16 mm in diameter) soaked in either 1 M LiTFSI in DOL/DME (50/50 vol%) or 1 M LiPF 6 in EC/DEC (50/50 vol%) was used as separator.
Cyclic voltammetry (CV) and galvanostatic measurements were performed in a lithium metal coin cell using a multi-channel Potentiostat (Biologic, VMP3) or an battery cycler (Neware), respectively. For the symmetric all-organic batteries, the MTA-MPT-based electrodes were both used as anode and cathode and Glass microfiber filter soaked in 1 M LiTFSI in DOL/DME (50/50 vol%) was used as separator.

Monomer salt synthesis
1 eq (128.04 mg, 0.3752 mmol) mellitic acid (MA) was dissolved in 40 mL deionized H 2 O whilst stirring at room temperature. 1.5 eq (128.40 mg, 0.5261 mmol) phenothiazine was added in the solid state to the transparent MA-solution whilst vigorously stirring. The reaction mixture was heated up to 60 °C and stirred for approximately 4 hours. A green monomer salt (MS) powder was obtained, which was filtered over paper (pore size 5-13 µm) and dried overnight at 55 °C in an oven.

Synthesis of dual redox-active porous polyimide
DA-MPT (1.5 eq., 0.3 mmol) and MTA (1 eq., 0.2 mmol) were weighed into a microwave reactor (Anton Paar, G30) and suspended in 10 mL degassed H 2 O. After adding a stirring bar, the reactor was closed with a cap including a PTFE septum and placed in the Anton Paar Monowave 400. The vessel was heated as fast as possible to 180 °C, stirred with 600 rpm, and held at that temperature for 1 h. Afterwards, the reactor was cooled down to 70 °C and the dark blue precipitate was collected via filtration. The product was washed using a soxhlet extraction method with tetrahydrofuran to remove unreacted monomers and small oligomers. Finally, the product (dark blue powder,

Pore size distribution and hypothesis for pore sizes
The PSD of PI(MTA MPT) is displayed below, evincing two populations around at ca. 0.5 nm and 0.8 nm: is not crystalline, but rather semicrystalline. Hence, significant disordered such as at aryl-N imide axes is expected (indicated by red arrows). Furthermore, the phenothiazine moiety is not planar, but kinked (see C: shown is an excerpt from the crystal structure of a Nethyl-phenothiazine containing Ni complex, extracted from the Cambridge crystallographic data centre CCDC, Refcode: 700928). For both the expected aryl-N axis disorder and the kinked structure of the phenothiazine segment, the network is expected to not be planar but "wavy"/kinked. As displayed in D: The N-N distance (d N-N ) in the phenothiazine moiety can be calculated as 9.981 Å (hypothesis: The distance corresponds roughly to the C3-C30 distance in CCDC Refcode: 700928). We expect N-N distance of the mellitic imide subunit to correspond to the O-O distance of mellitic trianhydride (extracted from CCDC Refcode 135999), i.e., 5.907 Å. Fixing the angle C at 120 ˚(which would correspond to a flat pore), we obtain a pore diameter of ca. c= 1.4 nm. For an Supporting information 14 interpenetrated structure -such as shown in E -two pore size populations at around 0.5 nm and 0.8 nm fit relatively well.